Intermittant problems can be very difficult to find. It may or may not have the self check feature---when it goes out if the power light goes off:
On the front of the set press both the menu and input (device) buttons at the same time and hold them for at least 5 seconds and then release both; power light (if this model has the self check) should blink a two digit error code like 2 blinks, a pause and 2 blinks---it will repeat it at least 5 times.
If you get an error code let me know what it is and I will tell you what it means.
If no error code let me know what happens when it goes out:
1) Is there sound?
2) Does the power light stay on or go off?
3) Is there any pattern to problem? (more when on after a while or when first turned on etc.)
I know these sets well and the more you can tell me the better the chance I can help you.
SD TECH